Pritchard had been closing in on a move to Yorkshire in recent days and, following discussions between the two clubs, a deal has now officially been struck.
The former England Under-21s international, who came through Tottenham Hotspur's academy ranks, has been handed the number 21 shirt at his new club.
Speaking to Huddersfield's official website, delighted boss David Wagner said: "I'm very happy to be able to welcome Alex to Huddersfield Town. He is exactly the type of player that we like to work with at this club.
"Alex is a young, British player who arrives as one of the top performers in the Sky Bet Championship and with a desire to prove himself in the Premier League.
"Now we will work hard with him to give him every possibility of being a success for us in the top division. I'd like to thank Dean Hoyle and the board for being as supportive as ever in this transfer window and making this transfer possible."
Pritchard spent seven years with Tottenham - five of those at senior level - before quitting to join Norwich on a permanent deal in 2016.